Cemented carbide materials are well known for their unique combination of properties of hardness, strength, and wear resistance and have accordingly found extensive use in mining tool bits, metal cutting and boring tools, metal drawing dies, wear resistant machine parts and the like. It is known that the wear resistance of cemented carbide materials may be enhanced by the application of thin coatings of a highly wear resistant material such as titanium carbide or aluminum oxide. These coated carbide materials are finding increasing commercial utility for certain cutting tool and machining applications.
Economic pressures for higher productivity in machining applications are placing increasing demands upon the performance of cutting tool materials. To achieve high productivity in machining, a tool must be able to cut at high speeds. At cutting speeds exceed 1500 surface feet per minute (sfpm), the high temperature strength and chemical inertness of a cutting tool material become more and more important. The usefulness of cemented carbide cutting tool materials (the predominant material used in cutting tools today) has been extended to applications requiring cutting speeds of about 1500 sfpm by coating such tools with aluminum oxide. For cutting speeds in excess of 1500 sfpm, cemented carbide tools encounter problems associated with loss of strength. Tool nose deformation, which affects dimensional tolerance in the workpiece and contributes to shorter tool life, becomes a problem at high cutting speeds with cemented carbide tools.
Conventional ceramic cutting tools overcome many of these disadvantages but have some limitations relating to their lower impact strength and fracture toughness. This is especially true of many alumina-based conventional ceramic cutting tools. Silicon nitride-based ceramic cutting tools have significantly higher impact strength and fracture toughness, but can exhibit lower than desired chemical inertness when employed in cutting long-chipping metals such as steel.
U.S. Pat. No. 4,252,768 to Perkins, et al. discloses abrasion resistant nozzles fabricated by distributing hard refractory metal carbides in a binary solid solution of a ceramic oxide and silicon nitride, preferably a ternary solid solution of a ceramic oxide, silicon nitride and aluminum nitride.
U.S.S.R. Pat. No. 537,986 to Gnesin discloses a ceramic material containing silicon nitride, metal carbide, and magnesium oxide or aluminum oxide, being exceptional in a way that, in order to increase the hardness and wear resistance, it contains as a form of metal carbide, titanium carbide.
A wide variety of silicon nitride based materials containing aluminum oxide and other additional oxides constitute a group of modified silicon aluminum oxynitrides. These materials have relatively low hardness which limits their usefulness as cutting tool materials.
In general, wear resistance of a tool depends upon both its resistance to abrasion wear (defined to a great extent by hardness and fracture toughness) and its resistance to chemical or diffusional wear. The wear mode of the tool depends upon the nature of the workpiece. In machining short-chipping materials such as gray cast iron, for example, tool wear is controlled predominantly by abrasion wear resistance, while in the machining of long-chipping materials such as steel, resistance to chemical wear becomes more important.